Generally, in an image guide fiber including a plurality of cores and a clad which is common to the plurality of cores, higher pixel density equates to greater image resolution, and higher core occupancy area ratio equates to greater brightness. Reducing core diameter for higher pixel density decreases core occupancy area ratio. Accordingly, to increase both pixel density and core occupancy area ratio for improving image quality, it is effective to reduce the thickness of the clad between adjacent cores (the distance between adjacent cores). However, too small a thickness of the clad between adjacent cores tends to cause crosstalk of transmitted light, possibly hindering the sufficient improvement of image quality. The minimum thickness of the clad between adjacent cores that is sufficient to fully prevent crosstalk depends on the numerical aperture NA of the image guide fiber. A higher numerical aperture NA makes it possible to reduce the minimum thickness the clad between adjacent cores that is sufficient to fully prevent crosstalk.
One conventionally proposed method for increasing the numerical aperture NA is to make the image guide fiber of multi-component glass (as described in, for example, Patent Literature 1).